Single-particle photoacoustic vibrational spectroscopy using optical microresonators

Vibrational spectroscopy is a ubiquitous technology that derives the species, constituents and morphology of an object from its natural vibrations. However, natural vibrations of mesoscopic particles—including most biological cells—have remained hidden from existing technologies. These particles are expected to vibrate faintly at megahertz to gigahertz rates, requiring a sensitivity and resolution that are impractical for current optical and piezoelectric spectroscopies. Here we demonstrate the real-time measurement of natural vibrations of single mesoscopic particles using an optical microresonator, extending the reach of vibrational spectroscopy to a different spectral window. Conceptually, a spectrum of vibrational modes of the particles is stimulated photoacoustically by the absorption of laser pulses and acoustically coupled to a high-quality-factor optical resonance for ultrasensitive readout. Experimentally, this scheme is verified by measuring mesoscopic particles with different constituents, sizes and internal structures, showing an unprecedented signal-to-noise ratio of 50 dB and detection bandwidth of over 1 GHz. This technology is further applied for the biomechanical fingerprinting of the species and living states of microorganisms at the single-cell level. This work opens up new avenues to study single-particle mechanical properties in vibrational degrees of freedom and may find applications in photoacoustic sensing and imaging, cavity optomechanics and biomechanics.